Hot-and cold-formed aluminum alloy

ABSTRACT

A component or semi-finished piece is made from a hot-form aluminium alloy of the following composition in wt. %: silicon 0.9-1.3, magnesium 0.7-1.2, manganese 0.5-1.0, copper less than 0.1, iron less than 0.5, chromium less than 0.25, titanium less than 0.1, zinc less than 0.2, zirconium and/or hafnium 0.05-0.2 and further unavoidable impurities, whereby the total amount of chromium and manganese and zirconium and/or hafnium is at least 0.4 by weight. The aluminium/silicon mixed crystals are present in addition to magnesium silicide precipitates.

This is a divisional of U.S. patent Ser. No. 10/499,755, filed Jun. 18,2004, which claims priority to German Patent Application No. 101 63 039,filed Dec. 21, 2001 through International Patent Application Serial No.PCT/EP02/14452, filed Dec. 18, 2002.

The present invention relates to a hot- and cold-workable aluminum alloyand to a method for manufacturing an aluminum component.

BACKGROUND

High-strength Cu- (for example, Al Mg Si 1 Cu 0.5) or Zn-containing,heat-treated Al semi-finished products and Al forgings do have highstatic strength levels, but their elongation at break is low. Therefore,in the case of a notch effect (for example, stone impact), this resultsin a low dynamic strength. Moreover, these alloys are susceptible tocorrosion so that expensive corrosion protection is required to avoidcorrosion pits having a notch effect. Since, for example, highlystressed, forged Al suspension components are always exposed to stoneimpact (notching) and corrosion, Cu-/Zn-containing Al materials are usedin these areas only in exceptional cases. Al Mg Si 1 alloys with higherductility or lower notch sensitivity, such as EN-AW 6082, are, in fact,corrosion-resistant because of their low Cu- and Zn-content; however,these alloys do not reach adequate strength levels.

Another disadvantage of such alloys is that during forming andsubsequent heat treatment, highly worked zones of forgings andsemi-finished products recrystallize, forming coarse grains. Acoarse-grained or brittle and less stable grain structure leads topremature failure of the Al component.

This is especially true when multiple forming operations are requiredduring forging, for example, to achieve high material yield. In the caseof multiple forming operations, the highest degree of deformationusually occurs only at the end of the forming process, and thus attemperatures between 390° C. and 450° C. so that the grain structurerecrystallizes during subsequent heat treatment. Even more problematicis the recrystallization behavior of cold-formed Al semi-finishedproducts that are subsequently heat-treated. For example, to producehigh-strength Al screws, cold-drawn wire or rods are used, which is/arethen cold-formed into a screw blank by upsetting and pressing. Duringsubsequent heat treatment, the grain structure is therefore highlysusceptible to recrystallization. The same is true for cold-forged Alwheels.

Unexamined German Laid-Open Patent Applications DE-OS 2 103 614 and DEOS 2 213 136 each describe an aluminum-silicon-magnesium alloy thatreacts in a recrystallization-inhibiting manner; however, these alloyshave insufficient strength, and the tendency of this alloy torecrystallize is still too high for cold-formed components or componentsundergoing multiple forming operations. The same is true for the knownalloy according to EN-AW 6082.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a component and amethod for manufacturing a component whose recrystallization-inhibitingactivity is improved over the prior art and which lead to higherstrength and corrosion resistance of the components.

The component or semi-finished product according to the presentinvention is made of an aluminum alloy having the following composition:silicon 0.9-1.3, magnesium 0.7-1.2, manganese 0.5-1.0, copper less than0.1, iron less than 0.5, chromium less than 0.25, circonium and/orhafnium 0.05-0.2. 

Advantageously, certain alloying constituents are present in thefollowing proportions: copper less than 0.05, iron  0.1-0.5, chromium0.05-0.2, zinc less than 0.05.

Moreover, the alloy may contain the elements zinc less than 0.2 titaniumless than 0.1.

Here, titanium is used for grain refinement, zinc may contribute to anincrease in strength. In addition, the alloy contains unavoidableimpurities that are attributable to the manufacturing process.

In an advantageous embodiment, the alloy has a silicon content ofbetween 0.9 and 1.7 percent by weight.

It is a further feature of the present invention that the alloyingelements manganese, chromium and circonium and/or hafnium all togetherrepresent a proportion of at least 0.4 percent by weight. Preferably,the proportion of these elements is higher than 0.6 percent by weight.These elements act as recrystallization inhibitors.

During homogenizing annealing, these elements, together with aluminum,form intermetallic dispersoids which anchor the grain boundaries and donot dissolve, or dissolve only to a small extent, even during furtherheat treatments. Because the dispersoids are anchored at their grainboundaries, the grains are prevented from growing to coarse grains, thuseffectively suppressing recrystallization. Circonium- andhafnium-containing dispersoids are particularly temperature-resistant,which has an inhibiting effect on the recrystallization at hightemperatures.

The alloy has a silicon content of from 0.9 to 1.3%. It has turned outthat a lower silicon content does not lead to the required strengthlevels. The silicon acts in combination with the magnesium in the formof precipitation hardening (heat treatment) which develops in the formof Mg2Si precipitates. Higher contents of manganese and chromium alsolead to precipitation hardening and an increase in strength.

Moreover, for solid solution hardening, i.e., the formation of an AlSisolid solution, it is expedient that there be an excess of silicon thatis not bound in Mg2Si precipitates. Therefore, the ratio of silicon tomanganese is preferably between 1.1:1 and 1.3:1, more preferably between1.16:1 to 1.24:1.

The alloy is particularly resistant to recrystallization both during hotand cold working, and intrinsically has high strength and a lowsusceptibility to corrosion, nearly independently of the manufacturingprocess. The low susceptibility to corrosion is primarily attributableto the low content of copper and zinc.

It is a feature of the method that the cast raw material of the alloy ishomogenized at temperatures between 420° C. and 540° C., preferablybetween 460° C. and 500° C. During this homogenization, the alloyingconstituents magnesium and silicon are finely distributed in thealuminum matrix and, moreover, the dispersoids form whose composition isbased on circonium or hafnium, manganese, chromium and/or iron.

It has turned out to be advantageous to homogenize the raw material forat least 4 hrs, particular preference being given to a homogenization of12 hrs.

In the further process, the raw material is formed into semi-finishedproducts at a temperature between 450° C. and 560° C. (for example, byextrusion or sheet rolling) and quenched, if necessary. Thesemi-finished products are preferably formed between 500° C. and 560°C., it being necessary to select, in each case, the highest temperaturepossible in order to avoid recrystallization nuclei. If necessary, thesemi-finished products are cut apart into workpieces that are suitablefor forming, and are either cold-formed once or multiple times orhot-formed into components or further semi-finished products, possiblymultiple times. The semi-finished products may also be machined in asuitable manner, for example, by turning or milling. Cold- orhot-forming or machining may be carried out within the scope of expertskills and may possibly include usual heat treatments.

The hot-forming of the semi-finished product is carried out attemperatures in the range of the usual solution treatment (between 440°C. and 560° C.). During the forming process, in particular, duringmultiple forming steps, care must be taken that the workpiecetemperature does not fall below the mentioned temperature, which wouldresult in coarse precipitations in the grain structure of the component.Accordingly, the forming process replaces the step of solutiontreatment, which has a considerable effect on the process costs andprocess duration.

The forming temperatures according to the present invention, which atthe same time imply a solution treatment, are higher than the usualforming temperatures, which results in a lower work hardening and thusin less formation of recrystallization nuclei in the grain structure.Thus, recrystallization is effectively suppressed, resulting in higherstrength levels and, above all, in a significantly higher elongation atbreak in highly worked areas.

After the forming process, the workpiece is preferably quenched inwater, thus freezing the grain structure. The desired increase instrength occurs during the subsequent artificial aging between 160° C.und 240° C.

If the composition meets the alloy specifications, the aluminumcomponent according to the present invention has a tensile strength ofat least 400 MPa and a minimum breaking strain (A5) of 10%. Componentsof this kind are preferably used as tension rods or other suspensioncomponents, sections, bolts, screws, or wheels.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the present invention is explained in more detail withreference to two examples. The process procedure on which examples 1 and2 are based is shown in the drawing, in which:

FIG. 1 shows a flow chart of a method of manufacturing a component orsemi-finished product according to the present invention.

DETAILED DESCRIPTION

According to FIG. 1, cast raw material, provided at block AA, ishomogenized at a temperature between 420° C. and 540° C. at block BB,and formed into semi-finished products, such as by pressing or rollingat a temperature between 450° C. and 560° C. at block CC. Then, thesemi-finished product is heated to a solution treatment temperature of440° C. and 560° C. and hot-formed, possibly multiple times, at thistemperature at block DD. The forged semi-finished product is thenquenched, for example in air or water, and artificially aged, forexample, at a temperature between 160° C. and 240° C. at block EE toarrive at the component or semi-finished product represented at blockFF.

EXAMPLE 1

An alloy melt having the composition in percent by weight: silicon 1.2,magnesium 1.0, manganese 0.5, copper 0.05, iron 0.2, chromium 0.2,titanium 0.05, zinc 0.1, circonium 0.2,is cast in ingots. The ingots are homogenized at a temperature of 480°C. for 12 hrs. In the next process step, the ingots are pressed intoround rods (=semi−finished product) at a temperature of 500° C. Theround rods are quenched and cut apart into workpieces having a length ofabout 20 cm.

The workpieces are heated to a temperature of 530° C. and formed intotension rods in several forging operations (=forming process). Duringforging, the workpiece temperature does not fall below 440° C. Thetension rods are quenched in water and artificially aged at 200° C. for4 hrs. The tension rods have a tensile strength of more than 400 MPa andan elongation at break (A5) of more than 13% both in the region of acentral rod and in the region of a large eye which usually has a highdegree of recrystallization due to the high degree of deformation.

EXAMPLE 2

Analogously to Example 1, cast ingots are homogenized and subsequentlyrolled into sheets (=semi−finished product) at a temperature of 500° C.Round workpieces are punched out from the sheets and formed into wheelsin several steps.

1-11. (canceled) 12: A method for manufacturing a component orsemi-finished product, the method comprising: homogenizing a cast rawmaterial at a temperature between 420° C. and 540° C.; forming the rawmaterial into a shaped part at a temperature between 450° C. and 560°C.; heating the shaped part to a solution treatment temperature between440° C. and 560° C.; hot-forming the shaped part at the solutiontreatment temperature so as to form a forged part; quenching the forgedpart; and artificially aging the forged part at a temperature between160° C. and 240° C. so as to create the component or semi-finishedproduct. 13: The method as recited in claim 21, wherein the quenching isperformed in at least one of water and air. 14: The method as recited inclaim 21, wherein the hot-forming is performed multiple times. 15: Themethod as recited in claim 21, wherein the homogenizing is performed forat least four hours. 16: The method as recited in claim 21, wherein thehomogenizing is performed for twelve hours.